How to Unmarshall data to JSON with unknown field types

Question

I have these 'structures'

type Results struct {
    Gender string `json:"gender"`
    Name   struct {
        First string `json:"first"`
        Last  string `json:"last"`
    } `json:"name"`
    Location struct {
        Postcode int `json:"postcode"`
    }
    Registered struct {
        Date string `json:"date"`
    } `json:"registered"`
}

type Info struct {
    Seed    string `json:"seed"`
    Results int64  `json:"results"`
    Page    int64  `json:"page"`
    Version string `json:"version"`
}

type Response struct {
    Results []Results `json:"results"`
    Info    Info      `json:"info"`
}

I' making a request to an external API and converting data to a JSON view. I know in advance the types of all fields, but the problem occurs with the 'postcode' field. I'm getting different types of values, which is why I get a JSON decoding error. In this case, 'postcode' can be one of three variants:

1. string ~ "13353"
2. int ~ 13353
3. string ~ "13353postcode"

Changing the postcode type from string to json.Number solved the problem. But this solution does not satisfy the third "option".

I know I can try to create a custom type and implement an interface on it. Seems to me the best solution using json.RawMessage . It's the first I've faced this problem, So I'm still looking for an implementation of a solution to this and reading the documentation.

What's the best way solution in this case? Thanks in advance.

Answer 1

Declare a custom string type and have it implement the json.Unmarshaler interface.

For example, you could do this:

type PostCodeString string

// UnmarshalJSON implements the json.Unmarshaler interface.
func (s *PostCodeString) UnmarshalJSON(data []byte) error {
    if data[0] != '"' { // not string, so probably int, make it a string by wrapping it in double quotes
        data = []byte(`"`+string(data)+`"`)
    }

    // unmarshal as plain string
    return json.Unmarshal(data, (*string)(s))
}

https://play.golang.org/p/pp-zNNWY38M